From prior art (EP 0 468 582) there is already known an X-ray diffractometer apparatus comprising optical means for measuring the reflectivity of a solid specimen. This apparatus is used for determining structural parameters of multilayer mirrors operating in the X-ray domain and formed by two-layer stacks of materials having a high and a low refractive index. This apparatus enables comparison of the parameters calculated on the basis of the reflectivity curve and the parameters known from classification tables.
The cited document discloses a control system for selecting the appropriate optical filters from filters having different absorption coefficients and for triggering their introduction into the path of the X-ray beam reflected by a structure to be tested. Thanks to this system, the reflected intensity can be measured by means of a proportional counter in such a manner that the latter never leaves its linear operating range, regardless of the intensity values. A proportional counter is to be understood to mean herein a device which contains gases which can be ionized by the photon flux to be detected, and which supplies a signal in the form of a voltage which is proportional to the number of photons received.
The known apparatus comprises a system for applying the output signal of the proportional counter to a device for processing this signal in which a computer program enables interlinking of the various parts of the reflectivity curve obtained in a relation with each of the filters of the controlled system. A smooth reflectivity curve is thus obtained.
This reflectivity curve, established at a constant wavelength .lambda..sub.RX of an X-ray source X and as a function of the variations of the glancing angle .theta., decreases strongly between the so-called critical value .theta..sub.c, at which the reflectivity equals 1, and values where the glancing angle is still small, i.e. smaller than or approximately equal to 4.degree. (degrees). Therefore, a controlled system of filters is used so that all intensity measurements can be carried out without departing from the linear range of the proportional counter. This curve also exhibits a given number of marked maxima of the reflected intensity which appear for specific glancing angles. The values of the glancing angles .theta. corresponding to the maxima satisfy Bragg's law:
2d sin .theta.=k.lambda.(where k=constant=an integer), and enable calculation of the period D of the layers of a periodic multi-layer system.
This determination, however, corresponding to the orders of the reflection, should also take into account the nature of the materials of the layers. Therefore, according to the teaching of the cited document, the pitch D of the periodic layers of multi-layer mirrors is determined, on the basis of the smoothed reflectivity curve, by comparison with theoretical curves constructed on the basis of classification tables.
Nevertheless, the device can be used only in cases where the reflectivity curve exhibits maxima which can be distinguished, i.e. sufficiently marked maxima, to enable exact measurement of their position in respect of glancing angle value. This method is thus restricted to the multi-layer structures formed by periodic stacks of layers whose material compositions are known and whose refractive indices deviate substantially, as well as to the multi-layer mirrors of known structure which do not have a large thickness and in which the period is not too small.